Saw oscillator with attenuator for protecting saw element

ABSTRACT

An oscillator with a surface acoustic wave filter connected in its feedback path. The oscillator includes an attenuator in the feedback path for attenuating to a predetermined level an oscillating signal fed back to the surface acoustic wave filter from an amplifier used to excite the oscillation. The attenuation is sufficient to prevent damage to the acoustic wave filter.

This application is a continuation, of application Ser. No. 06/848,962,filed Apr. 7, 1986 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oscillator, and more particularly toan oscillator with a surface acoustic wave filter in its feedback path.

2. Description of Prior Art

Recently there have been developed CATV systems which use a tuneradapted for receiving a plurality of broadcast channels. These tunersuse a so-called up/down heterodyne tuner which is better able toeliminate IF interruption. The up/down heterodyne tuner first convertsthe carrier frequency of the received signal to an intermediatefrequency (IF) higher than the carrier frequency, and then converts theIF to a predetermined frequency lower than the carrier frequency.

Up/down heterodyne tuners adapted for CATV systems are generallyconstructed as shown, for example, in FIG. 1. In FIG. 1, broadcastsignals (input signals) with predetermined carrier frequencies among afrequency band ranging, e.g., from 50-550 MHz are provided to an inputterminal 1 from a receiving antenna (not shown). The input signals areintroduced to a first mixer 3 through an input circuit 2. In first mixer3, the input signals are converted from their carrier frequencies to apredetermined intermediate frequency (IF) by using a first localoscillation signal applied from a first local oscillator 4 to firstmixer 3. The IF signal may be, for example 612.75 MHz, which is higherthan the highest frequency, 550 MHz, of the frequency band. The IFsignal is then input to a first IF filter 5, an IF amplifier 6 and asecond IF filter 7, where its level and tuned characteristic areadjusted to the desired level and characteristic by the circuit. The IFsignal output from second IF filter 7 is input to a second mixer 8. Insecond mixer 8, the IF signal is converted from 612.75 MHz to afrequency, for example, 61.25 MHz, which is lower than the IF and whichcorresponds to one of the predetermined channels set for televisionreceivers. The frequency conversion in the second mixer 8 is performedby a differential operation between the IF signal and a signal appliedfrom a second local oscillator 9 to second mixer 8.

The signal output from the second mixer 8 is introduced to an outputterminal 11 of the up/down heterodyne tuner through an output circuit10. The output signal on output terminal 11 is supplied to a televisionreceiver (not shown) as its so-called antenna input signal.

In the above described up/down heterodyne tuner, the selection of adesired channel is made by varying the frequency of first localoscillator 4. The output signal of this up/down heterodyne tuner isassigned to a frequency corresponding to any one of channels 1, 2, 3 and5 in Japan, or channels 2, 3, 4 and 5 in the U.S.A. and Korea. Theselection of the frequency accords to the frequency of the output signalof second local oscillator 9. Therefore, the output of second localoscillator 9 is varied in frequency in a band of 704-790 MHz in Japan,or 668-696 MHz in the U.S.A. and Korea, in general.

This up/down heterodyne tuner for CATV systems uses a Colpittsoscillator as second local oscillator 9. Colpitts oscillators containL/C resonance circuits comprised of an inductor L and a capacitor C in afeedback path for feeding back an oscillation output of the oscillatorto its input side with a prescribed phase relation. However, a Colpittsoscillator of this type has a disadvantage in that its output frequencyeasily fluctuates as a result of changes in temperature and humidity orthe bias voltage supplied to the circuit. Therefore, the outputfrequency of the up/down heterodyne tuner for CATV systems becomesunstable when an L/C resonance circuit is used for its second localoscillator 9, and interferes with broadcast reception by the televisionreceiver.

Many attempts have been made to improve the stability of the outputfrequency of the up/down heterodyne tuner. In these attempts, anoscillator using a surface acoustic wave filter (referred to as a SAWfilter hereinafter) in its feedback path has been developed, so that anup/down heterodyne tuner with an oscillator of the described type as itssecond local oscillator 9 is able to produce an oscillation output witha relatively stable frequency. The SAW filters are constructedprincipally as shown in FIG. 2. That is, interdigital transducerelectrodes (IDT electrodes) 20a and grating reflectors (GRs) 20brespectively, made of, e.g., aluminum film, are laid on a base 20c,e.g., made of the lithium-tantalate LiTaO₃ for mounting IDT electrodes20a and GRs 20b and transmitting acoustic waves therebetween.

Oscillators using the SAW filter create a problem in that the SAW filteris itself apt to be easily damaged by the oscillation signal in afeedback path. This problem is caused by the energy of a surfaceacoustic wave in the SAW filter concentrating and accumulating onspecified portions of the SAW filter. The surface acoustic wave ariseson base 20c as a standing wave, as illustrated by solid lines shown inFIG. 2. In FIG. 2, the solid lines also show the distribution ofdisplacement of base 20c due to the standing wave, while the dottedlines show the distribution of mechanical strain due to the standingwave in base 20c. That is, the energy of the surface acoustic wave,which is sometimes as high as the exciting power for the oscillator, isaccumulated in the SAW filter due to the standing wave. The energy isconcentrated on the surface of base 20c of the SAW filter. An excessiveconcentration of energy causes a so-called migration phenomenon in IDTelectrode 20a or GRs 20b fixed on the surface of base 20c of the SAWfilter. The migration phenomenon is the phenomenon that voids orhillocks occur in metal films like electrodes formed on dielectricbases, as known, e.g., in the field of semiconductor device techniques.It is believed that the voids or hillocks occur as a result of so-calledmetal fatigue phenomenon having excessively advanced due to repetitionof mechanical strains in metals.

In the SAW filters, the voids or hillocks increase the electricresistance of IDT electrode 20a or GRs 20b. Further, part of theoscillation signal energy is consumed in base 20c as Joule equivalentaccording to the piezoelectric effect in base 20c. The increasedelectric resistances and the consumed Joule equivalent energy furtherpromote the damage of the SAW filters.

Therefore, oscillators using SAW filters have been conventionally usedonly when the oscillators are not excited to a high energy oscillationstate. In other words, the oscillators must be operated at a low energystate. The exciting energy is very critical and it is difficult tomaintain the oscillation at low energy levels.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an oscillator in whicha SAW filter provided in a feedback path of the oscillator is protectedfrom damage due to the oscillation signal fed back to the SAW filter.

This and other objects are achieved in the oscillator of the presentinvention by attenuating the oscillation signal supplied to the SAWfilter. This oscillator includes:

1. an amplifier,

2. a feedback path for feeding back an output of the amplifier to itsinput end,

3. a surface acoustic wave filter (SAW filter) connected in the feedbackpath, and

4. an attenuator for attenuating the level of the output applied to theSAW filter.

Additional objects, advantages, and features of the present inventionwill further become apparent to persons skilled in the art from a studyof the following description and of the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of an up/down heterodynetuner for CATV systems;

FIG. 2 is a diagrammatical drawing for explaining transmissions ofsurface acoustic waves on a SAW filter;

FIG. 3 is a block diagram showing the preferred embodiment of theoscillator according to the present invention;

FIG. 4 is a level diagram showing signal levels at circuit positionsindicated in the circuit of FIG. 3;

FIGS. 5, 6, 7, and 8 are equivalent circuit diagrams for determiningconstants of some electrical components in the oscillator according tothe present invention;

FIG. 9 is an equivalent circuit diagram of the oscillator shown in FIG.3;

FIG. 10 is a circuit diagram showing a practical circuit construction ofthe oscillator according to the present invention; and

FIG. 11 is a Smith-chart for explaining the operation of the circuitshown in FIG. 10, in comparison with a conventional oscillator.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described in detail with reference tothe accompanying drawings, namely, FIGS. 1 to 11. Throughout thedrawings, like reference numerals and letters are used to designate likeor equivalent elements for the sake of simplicity of explanation.

Referring now to FIG. 3, there is shown a block diagram of an embodimentof an oscillator according to the present invention. In FIG. 3, anamplifier 21 for exciting the oscillation is provided with itsoscillation output introduced to an output terminal 22 through acapacitor 23. The oscillation output of amplifier 21 is fed to afeedback path 24 with a SAW filter 20 and fed back to the input end ofamplifier 21 through feedback path 24. In feedback path 24, anattenuator 25 is provided between the output of amplifier 21 and SAWfilter 20. Further a buffer amplifier 26 is connected between SAW filter20 and the input of amplifier 21.

Attenuator 25 attenuates the oscillation output of amplifier 21 to apredetermined level so as not to damage SAW filter 20. On the otherhand, buffer amplifier 26 amplifies the output of SAW filter 20 tocompensate for the gain loss of the feedback signal in SAW filter 20.

FIG. 4 shows levels of the signal in feedback path 24 at predeterminedcircuit positions in FIG. 3. That is A, B, C and D in FIG. 4 indicatethe predetermined circuit positions in FIG. 3, i.e., the input end ofSAW filter 20, the input end of buffer amplifier 26, the input end ofoscillation exciting amplifier 21 and the output end of oscillationexciting amplifier 21. As seen from the level diagram of FIG. 4, thefeedback signal supplied to feedback path 24 is attenuated to about 7 dBat position A as described in detail later. The feedback signal furtherlowers to about 4 dB at position B due to the gain loss in SAW filter20. Buffer amplifier 26 raises the level of the feedback signal atposition B to about 15 dB so that it compensates for the gain loss inSAW filter 20. The feedback signal with the level of about 15 dB atposition C is applied to oscillation exciting amplifier 21. Oscillationexciting amplifier 21 raises the level of the feedback signal frombuffer amplifier 26 to about 17 dB. That is, oscillation excitingamplifier 21 raises the level by only about 2 dB, but oscillationexciting amplifier 21 operates in its saturation region so that itsufficiently amplifies the oscillation signal. Therefore, the oscillatorshown in FIG. 3 is able to supply an oscillation output with large powerto foreign circuits, e.g., second mixer 8 of the up/down heterodynetuner as shown in FIG. 1.

Oscillation exciting amplifier 21 functions to provide sufficientenergy, or power, to maintain the oscillation of the oscillator itselfand also supplies the power which is needed in foreign circuits likesecond mixer 8 in FIG. 1. The oscillation power is so large that itwould damage SAW filter 20 if supplied directly. However, attenuator 25,connected in feed-back path 24 prior to SAW filter 20, attenuates thepower of the oscillation signal provided to feedback path 24 to adesired level not damaging to SAW filter 20. That is, attenuator 25attenuates the oscillation signal of about 17 dB at position D to about7 dB. The level of attenuation is determined not only by the gain lossin SAW filter 20, but also must be at a level necessary to maintain theoscillation of the oscillator. In other words, the amount of attenuationprovided by attenuator 25 is determined from the following twoconditions which conflict with each other. One condition is formaintaining the oscillation of the oscillator, and the other is forpreventing SAW filter 20 from being damaged due to the excessive energyof the feedback oscillation signal.

Referring now to FIGS. 5 through 9, some conditions for maintaining theoscillation of the oscillator will be discussed in detail hereinafter.The relation of attenuator 25 to the oscillation loop of the oscillatorwill be also described.

First, the description will assume that buffer amplifier 26 iseliminated from the oscillator shown in FIG. 3, for convenience's sakeof explanation, there being nothing novel or unknown regarding thebuffer amplifier. FIG. 5 shows an equivalent circuit for the oscillatorshown in FIG. 3. In FIG. 5, SAW filter 20 is represented by two sections20L and 20G. One section 20L is equivalent to the load impedance ofoscillation exciting amplifier 21 and it is comprised of capacitance Coand resistance RL. The other section 20G is equivalent to the signalsource impedance for oscillation exciting amplifier 21 and is comprisedof capacitance Co and resistance RG. Further, it is assumed thatmatching circuits M1 and M2 are provided at both the input and outputends of oscillation exciting amplifier 21, respectively. Matchingcircuits M1 and M2 respectively match with load impedance RL//Co andsignal source impedance RG//Co and provide predetermined phase shiftsbetween the signals at both the input and output ends of SAW filter 20,respectively. Provided the amplifying gain of amplifier 21 is higherthan the gain loss of the SAW filter by about 6-8 dB, the oscillation inthe oscillator will be maintained in a stable state. Further, if thecondition RL+RG>Rm (where RM is the equivalent resistance of Saw Filter20) is satisfied, the influence of mismatching the oscillation excitingamplifier 21 and both load and signal source impedances RL//Co, RG//Corepresented by equivalent resistance Rm of SAW filter 20 is reduced.

If the oscillator is used as second local oscillator 9 in the up/downheterodyne tuner shown in FIG. 1, oscillation exciting amplifier 21 mustbe matched with the input impedance ZM of second mixer 8 to whichoscillation exciting amplifier 21 is connected. Assuming that inputimpedance ZM of second mixer 8 is 110-j·300Ω when observed through aconnecting capacitance CM of, e.g., 1.5 pF, the oscillation output levelrequired in practice at the input end of second mixer 8 becomes about+12 dB (15.85 mW). By assuming that the peak voltage of the oscillationoutput is 70% of the source-drain voltage, Vds, i.e., 7.59 V, a voltagegenerated across the source and the drain electrodes of an amplifyingmeans, e.g., an FET constituting oscillation exciting amplifier 21,equivalent load resistance RL is obtained as follows ##EQU1##

The current value of the oscillation output IL is given as IL=5.97 mA atthat time.

Therefore, the bias condition of oscillation exciting amplifier 21 mustbe determined in calculating input impedance ZM of second mixer 8 as aload impedance for the oscillator.

The connection of oscillation exciting amplifier 21 to second mixer 8will next be discussed with reference to the oscillation's stability.When an FET 3Sk115 is used as the amplifying device of oscillationexciting amplifier 21, the y- parameters of the FET 3SK115 at afrequency around 674 MHz are as follows,

    Yi=0.557+j 3.65 ms

    yf=19.4-j·4.46 ms

    yr=0 001-j·0.07 ms

    yo=0.382+j·1.44 ms

Therefore, the admittance YL transformed from the input impedance ZM ofsecond mixer 8 is calculated as YL=1.12+j·3.41 ms.

Where, adding output admittance yo of FET 3SK115 to input admittance YL,the total admittance Yo becomes as follows,

    Yo=yo+YL=1.50+j·4.85 ms

Then, a stability coefficient C of Linvill's Stability-judge Equationwill be calculated as follows, ##EQU2##

As seen from the above equation, a relation 0<C<1 is established, sothat the connection of oscillation exciting amplifier 21 to second mixer8 will be stable.

Equivalent signal source impedance RG and equivalent load impedance RLmay be determined as follows. An electrical equivalent circuit for SAWfilter 20 is shown in FIG. 6. In FIG. 6, RM, Lm and Cm are electricalcoefficients equivalently representing the mechanical coefficients ofSAW filter 20. Co is the input and output capacitance as mentionedbefore.

First, for determining the frequency range BW of the oscillation signal,it is assumed that the accuracy of the center frequency is ±0.01%, and apractical tuning range is ±25% of the frequency range BW where theoscillation signal lowers up to 3 dB in level. Then the followingequation is obtained,

    0.25 BW=±0.00001×674 MHz=±67.4 KHz

From the above equation, the frequency range, BW=₂₇₀ KHz is obtained.

Including the Q of SAW filter 20 in considering equivalent loadimpedance RL in FIG. 6 as QL and assuming QL as follows,

    QL=QU·Rm/(Rm+RL+RG) and substituting following respective data into the above equation of QL,

QL=2500 (∵ QL=fo/BW=674 MHz/270 KHz=2500)

RM=150 Ω

QU=11,285

signal source and load impedance RG, RL are obtained as, RG=RL=260 Ω

In addition, phase lags of the oscillation signal at both the amplifyingdevice and SAW filter 20 must be compensated for. The compensation willbe discussed with reference to FIG. 7, another equivalent circuit of theoscillator. In FIG. 7, however, inductances Lo are connected to bothends of oscillation exciting amplifier 21 in parallel with signal sourceimpedance RG//Co and load impedance RL//Co for compensating the phaselags. As described before, the susceptance of the FET 3SK115 is -4.46ms, while the input and output susceptances of SAW filter 20,considering input and output capacitances Co each of 1.6 pF, are both-6.8 ms. The total susceptance of the circuit, then becomes-4.46+(-6.8×2) (=-18.06) ms. The phase lags caused by the susceptanceare compensated by inductances Lo in FIG. 7. Thus, an inductivesusceptance of +18.06 ms in total is required for eliminating thecapacitive susceptance, -18.06 ms. The reactance to be given by eachinductance Lo is calculated as follows,

    Lo=1/[(18.06×10.sup.-3)/2]=110.75 Ω

Each inductance Lo is, therefore, calculate as Lo=26 nH (Lo=110.75/2×674 MHz=26 nH), for compensating the phase lags.

In the above example, the compensation of the phase lags of theoscillation signal at oscillation exciting amplifier 21 and SAW filter20 is made by inductances Lo. But it is achieved by other ways, e.g.,π-shape matching circuits π 1 and π2 as shown in FIG. 8. Thecompensation by π-shape matching circuits as shown in FIG. 8 has theadvantage that respective elements in the π-shape matching circuits areable to be fixed, independently from the input and output impedances.

As described above, both conditions, the load matching condition of theoscillator to its load circuit, e.g., second mixer 8 in FIG. 1, and thephase lag compensation condition for oscillation exciting amplifier 21and SAW filter 20, have been obtained in the equivalent circuits shownin FIGS. 5 to 8.

The allowable energy of the oscillation signal fed back to SAW filter 20will be discussed with reference to FIG. 9, an equivalent circuit of theoscillator. In FIG. 9, blocks 20 and 21, respectively, show electricalequivalent circuits of the SAW filter and the oscillation excitingamplifier similar to FIG. 7. Capacitances Co connected at both ends ofSAW filter 20 represent its input and output capacitance. Block 26connected between the output end of SAW filter 20 and oscillationexciting amplifier 21 represents an electrical equivalent circuit of thebuffer amplifier. Resistor RA and inductor LA connected in seriesbetween the output end of oscillation exciting amplifier 21 and SAWfilter 20 represent attenuator 25 and matching circuit M2 as shown inFIG. 3, respectively. Inductance L01 and L02 provided at the output endsof both SAW filter 20 and oscillation exciting amplifier 21 compensatethe respective phase lags of the oscillation signal in SAW filter 20 andthe oscillation exciting amplifier 21. The values of inductances L01 andL02 are set to the value of inductance Lo, calculated with reference toboth the susceptance of the amplifying device, e.g., the FETconstituting oscillation exciting amplifier 21, and the susceptancebased on input and output capacitances Co of SAW filter 20 as mentionedbefore. Inductance L10, connected to the input end of oscillationexciting amplifier 21, is provided for matching the phase discrepencybetween buffer amplifier 26 and oscillation exciting amplifier 21.Attenuator 25 includes resistor RA for attenuating the oscillationsignal fed back to SAW filter 20 through feedback path 24. Inductor LAconnected in series with resistor RA functions as an 180° phase shifterfor feeding back the output of SAW filter 20 to its input end aftershifting the phase of the output, i.e., the phase of the oscillationoutput of oscillation exciting amplifier 21, 180° when SAW filter 20 isan inversed phase type filter.

Referring now to FIG. 10, there will be described in detail a practicalconstruction of the oscillator shown in FIG. 3. In FIG. 10, oscillationexciting amplifier 21 is comprised of MOSFET 21a as its amplifyingdevice. The drain electrode of MOSFET 21a is connected to an outputterminal 22 of the oscillator through a capacitor 23. Output terminal 22is provided for the connection with, for example, second mixer 8 of theCATV up/down heterodyne tuner as shown in FIG. 1. The drain electrode ofMOSFET 21a is further connected to attenuator 25 through matchingcircuit M2 consisting of a series circuit of inductor LA and a capacitorCA. Attenuator 25 includes a series resistor 25a, a bypass capacitor 25band a bypass resistor 25c. Series resistor 25a operates to attenuate theoscillation signal fed back to SAW filter 20 together with bypasscapacitor 25b. Bypass resistor 25c makes a discharge path for bypasscapacitor 25b. Due to both series resistor 25a and bypass capacitor 25b,the oscillation signal applied to SAW filter 20 is suppressed to thelevel shown in FIG. 4, position B.

Buffer amplifier 26 includes a gallium-arsenide transistor (GaAstransistor) 26a as its amplifying device. The output end of SAW filter20 is connected to the one gate of GaAs transistor 26a through matchingcircuit M1 comprised of a series capacitor CB and a bypass inductor LB.The drain electrode of GaAs transistor 26a is connected to the one gateof MOSFET 21a through a phase adjusting circuit 27 comprised of a bypassinductor operating as inductance L10 shown in FIG. 9 and a seriescapacitor 27a. MOSFET 21a in oscillation exciting amplifier 21 and GaAstransistor 26a in buffer amplifier 26 are connected at their respectivesource electrodes and the other gate electrodes to bias circuits.

In the oscillator shown in FIG. 10, oscillation exciting amplifier 21produces an oscillation signal of sufficient power by saturation zoneamplification of MOSFET 21a and then provides the oscillation signal to,e.g., second mixer 8 in the CATV up/down heterodyne tuner shown inFIG. 1. The oscillation signal provided to feedback path 24 is, however,attenuated sufficiently by attenuator 25 so that SAW filter 20 isprotected from damage by the energy of the oscillation signal fromoscillation exciting amplifier 21.

GaAs transistor 26a in buffer amplifier 26 acts to reduce the load ofSAW filter 20 from the oscillation signal. GaAs transistor 26a has anextremely small input capacitance in comparison to any other knownamplifying devices. Therefore, inductance, LB in matching circuit M1,connected to the output end of SAW filter 20, may be set larger ininverse proportion corresponding to the small input capacitance of GaAstransistor 26a. The large inductance LB serves to reduce the power ofthe oscillation signal applied to SAW filter 20. In addition, the largeinductance LB allows SAW filter 20 to operate for the oscillation signalwith a higher frequency.

Referring now to FIG. 11, the Smith-chart in 50 Ω normalization, thecalculation of output impedance Z of SAW filter 20 will be described.The output impedance characteristic of a GaAs transistor, e.g.,3SK121-Y, is indicated by P1 in the chart. From the coordinate system ofthe Smith-chart corresponding to P1, the input resistance R1 of 55 Ω andthe input reactance X1 of 350 Ω at the signal frequency of 710 MHz areread. The output impedance Z1 of GaAs transistor 3SK121-Y is obtained asZ1= 354.3 Ω. The output voltage V1 and the current 11 at the time arealso obtained as V1=471.3 mV and I1=1.33 mA. The current I1 of 1.33 mAis smaller than the rated current of GaAs transistor 3SK121-Y, 2.0 mA,so that SAW filter 20 is protected from being damaged, by theoscillation signal.

For a comparison to the above, a MOSFET, 3SK115FA-1, g used in bufferamplifier 26 will be described. The output impedance characteristic ofMOSFET 3SK115FA-1 is indicated by P2 in the chart. From the coordinatesystem of the Smith-chart corresponding to P2, the input resistance R2of 60 Ω . and the input reactance X2 of 95 Ω at the same signalfrequency of 710 MHz are read. The output impedance Z2 of MOSFET3SK115FA-1 is obtained as Z2=112.4 Ω. The output voltage V2 and thecurrent I2 at that time are also obtained as V2=265.0 mV and I2=2.36 mA.The current I2 of 2.36 mA is larger than the rated current, 2.0 mA.Therefore, SAW filter 20 will be damaged if an amplifying device with arelatively large input capacitance, e.g., a MOSFET, is used as theamplifying device in buffer amplifier 26.

As described above, the oscillator according to the present invention isable to suppress an oscillation signal fed back to an SAW filter in afeedback path of the oscillator to a sufficiently low level to preventdamage to the SAW filter. The present invention also is able to providean oscillator which operates stably at an oscillation signal with ahigher frequency which accompanies a relatively high energy signal.

In the above embodiment, attenuator 25 may be located at any location infeedback path 24, and is not limited to the position prior to SAW filter20. Oscillation exciting amplifier 21 and buffer amplifier 26 may bemade of two or more amplifying devices, respectively. Further, thepresent invention may be used widely for a general oscillator with anSAW filter in its feedback path, not being limited to the oscillator fora CATV up/down heterodyne tuner.

What is claimed is:
 1. An oscillator comprising:an amplifier means withan input and an output, said amplifier means exciting an oscillation andproducing an output signal at its output; a feedback path for feedingback said output signal of said oscillation exciting amplifier means toits input, said feedback path including:an inductor and an attenuatorconnected in series to the output of said amplifier for attenuating thelevel of said output signal and for shifting the phase of said outputsignal by 180°; a surface acoustic wave filter connected to receive thephase shifted output signal from said attenuator and inductor; a secondinductor connected between the output of said surface acoustic wavefilter and ground for phase compensation of the original output fromsaid surface acoustic wave filter; a buffer amplifier means connectedbetween the output of said surface acoustic wave filter and the input ofsaid oscillation exciting amplifier means; and a third inductorconnected on one end to the output of said buffer amplifier means and onthe other end to ground, wherein said buffer amplifier and said thirdinductor act to again shift the phase of said phase shifted outputsignal by 180°.
 2. An oscillator according to claim 1, wherein saidbuffer amplifier means is comprised of a gallium arsenide amplifyingdevice.